An electrical power distribution or transmission system comprises protection systems arranged to protect, monitor and control the functioning of devices forming part of the power system. The protection systems detect, among other things, short-circuits, over-currents and over-voltages in power lines, transformers and other parts of the power distribution system.
Protection equipment is used throughout the electrical power system for providing such protection and control. The protection equipment detects and isolates faults on, for example, transmission and distribution lines by opening circuit breakers, and thereafter restoring power flow after the fault has been eliminated. Alternatively, the protection equipment may be arranged to select an alternative route for the power flow upon detection of a fault.
Current differential protection is a relatively new and reliable method for protection of power networks. It is based on the idea of comparing currents on both sides of a protected zone or a protected unit. A protected unit or zone could be any part of the power network, for example a transmission line, transformer, generator, or a transformer station busbar.
FIG. 1 illustrates schematically the principle of current differential protection. Under normal operating conditions, the sum of all currents I1, I2 entering and leaving a protected unit 1, for example a transmission line, is equal to zero. If there is a fault on the protected unit 1, the summation of currents from the various ends will no longer be equal to zero. Under normal operating conditions secondary currents in current transformers 3a and 3b, connected between the protected unit 1 and current relays 2 and arranged to step-down the primary power system current, are also equal, i.e. i1=i2 and no current flows through the current relay 2. If a fault occurs within the protected unit 1 the currents are no longer equal and there is a current flowing through the current relay 2. The differential relay 2 then trips circuit breakers arranged at both ends of the protected unit 1 (not shown). The opened circuit breaker(s) will thereby isolate the faulty protected unit from the rest of the power network.
At present, local measurements are used to determine if the current transformer circuits are functioning. FIG. 2 illustrates a protected transmission line L. Common practice is to use two main protection devices for protection, for example current differential protection equipment 4 and distance protection equipment 5. In order to detect faults, the current differential protection equipment 4 utilizes measured values from its current transformer CT1 and also a reference value from a current transformer CT2 of the other main protection device 5. That is, measurement values from the local current transformers CT1 and CT2 are used, which requires the mixing of circuits of the two protection equipment 4 and 5. A drawback with this solution is that it is difficult to determine whether a detected fault originates in the actual protection equipment or in the reference protection equipment.
Further, it can be difficult to determine whether a fault that has occurred is a primary network fault or a secondary network fault. If a secondary fault has occurred, for example faults occurring within the current transformer circuits, it is most undesirable to trip a circuit breaker so that primary transmission lines are taken out of service unnecessarily, thereby interrupting supply of electric power to power users.
In view of the above, it would be desirable to provide an improved method for providing differential protection. In particular, it would be desirable to provide a method for providing differential protection supervision of current transformer circuits. Further, it would also be desirable to provide a corresponding differential protection system.